In Delhi, A Different Kind Of Heat Worries Slum Dwellers

“Can you feel the humidity here?” Usha asked me as she wiped her sweaty face. “We bathe at least three to four times in the night. My whole body has developed rashes.”

Usha is a domestic worker and a resident of Mukherjee Nagar in North Delhi. She lives in Nandlal jhuggi (slum), which is home to daily wage labourers, rickshaw pullers and home-based workers. The residents said that nights are tougher than days, as the heat is compounded by humidity and power outages.

Some 32.2% of Delhi’s population (including both rural and urban) stay in single-room dwellings, according to the Delhi Statistical Handbook 2023. Most of the urban slums in Delhi ​​have congested houses with poor ventilation, and sunlight doesn’t reach these shanties.

In 2023, the Intergovernmental Panel on Climate Change (IPCC) confirmed that one of the greatest hazards posed by near-term 1.5°C warming is the impact of “dangerous” humid heat waves on human mortality.

A study conducted by scientists from the Yale School of the Environment, published in Nature in April 2023, examined the combined impact of temperature and humidity on urban heat stress using observational data and urban climate model calculations. The researchers discovered that the burden of heat stress is influenced by the local climate, and that increased humidity can negate the cooling benefits typically provided by trees and vegetation.

What does humidity do to the slums?

The lane that leads to Usha’s room in Nandlal Juggi in Delhi’s Gopalpur village. Rapid urbanisation is worsening Delhi's microclimate, causing higher temperatures and reduced wind flow in densely populated slums.

This year, Delhi faced severe heat waves with temperatures that soared past 45°C on several days. The most vulnerable people--those who live in informal settlements--suffered the brunt of this extreme weather. People died from the heat, with media reports suggesting up to 275 fatalities due to heat-related illnesses. The actual number of heatwave deaths may be higher, experts say.

The Union health ministry and the India Meteorological Department issued advisories as heatwave conditions persisted in various parts of the country. Typically, it is considered a heat wave if the maximum temperature of a station reaches at least 40°C or more for plains, and at least 30°C or more for hilly regions.

But as rain lashed parts of Delhi-National Capital Region, temperatures dipped. On one such rainy day, I visited some slum settlements in Delhi and listened to the people talk of a different kind of heat--one that appears to escape notice because it is not measured and tracked.

Many houses in urban informal settlements have very poor ventilation, leading to extreme heat accumulation during the summer. Indoor temperatures can average between 35-40°C, making the severity of heat waves more pronounced inside slum houses compared to the outdoors.

In a room as small as the size of a double bed, Sahana, her husband and her two-year-old kid sleep, cook and take baths. Sahana is seven months pregnant. She spends a lot of her time sitting at the entrance of this square-shaped room, occasionally stepping out to sit under a huge peepal tree adjacent to an open drain. For cooling her home, apart from a ceiling fan, all that Sahana has is a small exhaust fan.

“What do we do to escape this? We can’t afford a cooler. This exhaust fan is also not helping now,” she says. Hers is a common story. In almost all houses in Nandlal Juggi and Taimur Nagar Juggi, people have resorted to exhaust fans to get some relief from the humidity.

Sahana poses with her two-year-old son outside her residence in Nandlal Juggi in Delhi’s Gopalpur village. She is seven months pregnant, and spends a lot of time sitting at the entrance of this square-shaped room, occasionally stepping out to sit under a huge peepal tree adjacent to an open drain.

Residents in Nandlal Juggi and Taimur Nagar have installed exhaust fans to get relief from humidity.

According to a 2017 study, the global climate is not just getting warmer but also more humid. In many parts of India, the combination of high temperatures and humidity proves especially hazardous to human health and well-being.

This combined scientific measure of heat and humidity is called the 'wet bulb temperature', which can reach alarming levels even when the actual temperature is moderate.

For example, a temperature of 30°C with 90% relative humidity results in a wet bulb temperature of an uncomfortable 29°C, as per this National weather service calculator. Relative humidity (RH) is a measure of how much water vapour is in a water-air mixture compared to the maximum amount possible.

Our survival depends on our bodies staying at a consistent temperature, as IndiaSpend explained in August 2017. Normally, our core temperature--from the top of the head to mid-chest, encompassing the brain, lungs and chest--is regulated at 37°C, and our skin at 35°C. Sweating helps us shed excess heat--sweat beads cool the skin, and evaporation of sweat gets rid of heat, restoring the equilibrium.

This process of thermoregulation occurs efficiently only when ambient air is favourable for us. Air can hold a limited quantity of water until it saturates. In dry air without much water content, your sweat evaporates quickly, causing cooling. In humid air that has much moisture already, sweat does not evaporate as easily, leading to overheating of the body.

Recorded at Zakir Nagar, a congested settlement in Delhi at 6.49 p.m. on July 4, the Apple weather app shows how humidity is making it feel hotter.

In April 2019, a study published in the journal Science of the Total Environment explored and contrasted heat exposure in low-income neighbourhoods with other areas in Delhi (India), Dhaka (Bangladesh) and Faisalabad (Pakistan). The study assessed outdoor microclimatic conditions using thermal indices instead of solely focusing on urban heat islands or air temperatures. The researchers proposed that heat action plans (HAPs) should incorporate thermal indices to include factors such as humidity in addition to temperature thresholds.

The case of Delhi

Nearly 1,800 unauthorised colonies house over 30% of Delhi's population of more than 20 million. These colonies have proliferated over the years, continuing to expand unchecked.

The problem of illegal colonies cropping up across the city came to the notice of the government for the first time in 1961, forcing it to consider regularising them. One year later, the Delhi government regularised the first batch of 103 such residential pockets.

Talking about ​​how regularisation helps an informal settlement (both from the legal and the habitability point of view), Aravind Unni, an urban researcher and activist explains that regularisation should involve land digitisation, providing property titles, and planning for overall improvements in settlements, including amenities, roads, water and services. However, this process has largely been de facto, offering protection and continued policies rather than proper planned development. Despite this, people welcome even partial regularisation as it provides security and increases the value of their housing.

Unni says, “The process by which the government decides which colonies will be regularised is frankly very random. It has a lot to do with who lives there, the land use, the older the settlement the better the chances and so on.”

He explains that though unauthorised colonies are not exempt from the building by-laws, there is no formal, well-thought-out plan for development in these areas. Development is mostly through formal sanctions approved at the Municipal Corporation of Delhi (MCD) level with fees, but often also controlled informally through bribes. The more formal the structure, the more formal the by-law process. Like urban villages, where by-laws don't apply and operations are almost entirely informal, slums are considered illegals.

“By-laws should apply to all, but due to the lack of formalisation of different types of settlements in our city, there is no effective way to implement them," Unni tells IndiaSpend.

IndiaSpend has reached out to Ashwani Kumar, commissioner, and Sivaprasad K.V., municipal secretary, at the MCD for comment. We will update this story when we receive a response.

Parmeela, a domestic worker, moved to Delhi some 25 years ago. Nandlal Juggi in Gopalpur village, where she lives, is one of the many non-affluent unauthorised colonies in Delhi.

Parmeela’s house has a room and a kitchen with a small common area. The room has a small fan. It feels claustrophobic, with zero ventilation. Parmeela told me that two people sleep in this room. “Can you believe someone sleeps here?” she asked me. “That was my husband, who was sleeping here without clothes.”

I sat in her room with five other women who were all either domestic workers or daily wage labourers. They talk about how their husbands feel sick while on their jobs out in the sun. Anita, another domestic worker, tells me that her husband, who runs an e-rickshaw, fainted while driving. He was quickly rushed home and returned to work the next day.

Parmeela and her neighbours sit in her room in Nandlal Juggi in Delhi’s Gopalpur village. The women speak about how their husbands feel sick while on their jobs out in the sun.

Kamala Devi, another woman from the group said, “We don’t sleep inside our rooms at night.” “Some people sleep on the roof and some just spend the night outside.” She told me that they are only able to get some sleep in the morning hours, by which time it is slightly less humid. But by then, it is time for work.

“My husband sleeps in front of the only small cooler we have, so that he can go to work with enough sleep,” Preeti Sharma, a resident of Taimur Nagar village, tells me.

The Delhi Heat Action Plan 2024-25 says that if humidity is high, a person can suffer from heat wave disorders even with the temperature at 37°C or 38°C. Normally, 37°C or 38°C dry bulb temperature is well under the human tolerable limits.

“Wet bulb temperature allows us to actually measure how we feel under a particular temperature. It can determine what both temperature and humidity does to our body,” says Chandra Bhushan, the president and chief executive officer of the International Forum for Environment, Sustainability and Technology (iFOREST), a New Delhi-based think-tank.

The moisture in the air and temperature together affect our body, he explains.

Bhushan says, “Since last year, IMD has started to announce something called the ‘feels-like temperature’, which is calculated by combining heat and humidity.”

‘Feels like temperature’ as recorded by the India Meteorological Department on July 12, 2024

“But IMD continues to declare heat waves along with mitigation and adaptation measures only based on dry bulb temperature, not accounting for humidity,” he adds.

A wet-bulb temperature of 35°C is widely used as the threshold for human survivability, but the wet-bulb temperature is not a particularly accurate metric for human heat stress, experts say. For a person in the shade, a more accurate metric is the ‘Heat Index’.

Delhi’s Heat Action Plan does not mention the word 'wet bulb temperature'. It says that to calculate the effect of humidity we can use 'Heat Index Values', which is a measure of how hot it really feels when relative humidity is factored in with the actual air temperature.

As an example, if the air temperature is 34°C and the relative humidity is 75%, the heat index--how hot it feels--is 49°C. The same effect is reached at just 31°C when the relative humidity is 100% The temperature vs humidity chart and the temperature actually felt is given in the plan as per the table below.

Temperature/ Humidity Index.
Source: Delhi Heat Action Plan 2024-25

Why India needs a ‘heat code’

“Right now, India needs a 'heat code' for cities and states that will account for both heat and humidity, while discussing heatwaves,” says Bhushan. “City heat action plans are not working. There is no finance, no institutional coordination.”

“The Delhi Heat Action Plan was, frankly, not implemented. It was notified in April, just before the heat season,” Bhushan told IndiaSpend.

IndiaSpend has reached out to Sushil Singh, special chief executive officer of the Delhi Disaster Management, authority for comment. We will update the story once we receive a response.

For every 1°C the planet warms, humidity rises by about 7% as water evaporates from our oceans and waterways.

In 2019, researchers from the Indian Institute of Tropical Meteorology evaluated nine climate models and found that the frequency and duration of heatwaves could start increasing as early as 2020, continuing to rise with further global temperature increases.

A 2017 study shows that if the global mean temperature is limited to 2°C above pre-industrial conditions, the frequency of severe heat waves will rise by 30 times the current climate by the end of the 21st century.

Consequently, all climate models suggest that heatwaves are likely to become a significant natural disaster in the near future.

In response, many cities and states have recognised the threat and created HAPs. The Union government is collaborating with 23 heat wave-prone states and over 130 cities and districts to develop and implement HAPs across the country.

“Despite these efforts, the overall national response remains significantly insufficient,” Bhushan says.

IndiaSpend has reached out to Rajendra Singh, member and head of the department at the National Disaster Management Authority (NDMA), and Safi Ahsan Rizvi, advisor (mitigation) at the NDMA for comment. We will update this story when we receive a response.

He explains that a heat code would consider both humidity and temperature when declaring heatwaves, and formulating mitigation and adaptation strategies. This code would include standard operating procedures for heatwave conditions, such as setting work-hour limits and implementing relief measures in public spaces and hospitals. Additionally, it would empower district administrations to declare emergencies, halt outdoor activities, and allocate resources for heat relief efforts.

The Government of India does not classify heat waves as natural disasters under the National Disaster Management Act of 2005. As a result, heatwaves are not eligible for support from the National or State Disaster Response Funds.

“There remains a lack of sufficient resources and capacity to effectively enhance resilience in cities and states,” Bhushan told IndiaSpend.

Health impacts

Heat waves caused at least 6,167 deaths across India between 2010 and 2018. Of these, 34% or 2,081 deaths were reported in 2015, the year with the deadliest heat waves, IndiaSpend reported in June 2020.

According to Dileep Mavalankar, former director of the Indian Institute of Public Health Gandhinagar, humidity affects the cooling mechanism in the body. The sweat does not evaporate due to high moisture. In such conditions, the body heats up and this might result in heatstroke in people with comorbidities.

Mavalankar says, “We actually don't know if wet bulb temperatures are higher in slums, that data has never come from a weather station. Weather stations are located only in open and spacious locations. They don't give micro-climate data. They only give a macro-level picture.”

Repeated exposure to high wet bulb temperatures can lead to heat stress and exhaustion. Extreme heat coupled with high humidity can affect human health after just a short period of exposure. High wet-bulb temperatures can affect the habitability of buildings.

“Unfortunately, India has no research to show what kind of heat load people are experiencing in the slums due to high wet bulb temperature,” Mavalankar says.

Building heatwave resilient houses in Delhi slums

Rapid urbanisation is changing the microclimate in Indian cities, leading to reduced wind flow and higher temperatures in densely populated areas.

Even though the Unified Building Bylaws for Delhi do not mention any environment-related guidelines, the Ministry of Environment, Forests & Climate Change notified ‘Environmental Guidelines for Buildings’.

Unni explains that some general guidelines for air ventilation and environment exist, but they need to be completely rethought and reworked. “The problem with our building by-laws is that they are too focused on the material--building / brick mortar, cars and so on--but not on the unbuilt, environmental aspects,” Unni says. “This needs to change.”

Delhi has 6,343 slums. “People living in slums and low-income communities are particularly heat vulnerable. Large percentage of their homes are far from optimal,” Delhi’s Heat Action Plan says.

The combination of extremely high temperatures with high humidity represents a greater threat to human health. Mavalankar says, “In humid conditions, even drinking water doesn’t help. There is not much one can do about humidity except for waiting for it to pass.”

Several non-profit organisations have come up with different small-scale pilot projects to build heatwave-resilient houses in Delhi slums. Indo Global Social Service Society (IGSSS) piloted a project to install agri-bio-panels, a heat insulator in four slums of Delhi. In all, 30 houses were selected for the heat insulator panel.

Proshin Ghosh, Lead, Climate Change Adaptation Programmes, IGSSS who was a part of the project, says, “In-house temperature became reduced 3-4°C on average in all beneficiary households.”

Agri-bio-panels, a heat insulator being installed in Delhi slums by IGSSS.

In the absence of good community infrastructure, a ‘Community Cooling Center’ could be a solution. Abhiyant Tiwari, the lead climate resilience and health consultant at the Natural Resources Defense Council (NRDC), says that NRDC, in partnership with Jodhpur Nagar Nigam and the non-profit Mahila Housing Trust, recently launched a pilot project for a community cooling centre in Jodhpur as part of the Jodhpur Heat Action Plan. This innovative cooling station, powered by solar panels, has mist fans, grass shades, and is surrounded by water sprinklers. It has the capacity to accommodate over 40 people at a time.

“This is a place where anyone can come to get relief from the indoor and outdoor heat,” Tiwari says. “These are portable solutions and a lot of R&D can happen on this. Multiple such solutions can be deployed by different cities.”

Unni says, “The effectiveness of these piloted solutions completely depends on how the users feel and if they have benefited. We're currently experimenting with these ideas, but once documented, they could be scaled up with government support and incentives.”

"Urban planning must include ventilation, building spacing, natural cooling, and tree planning,” Unni points out. “It's unfortunate that current plans are outdated and don't address heatwaves, climate change, or resilience.”